In a typical wireless communications system, a base transceiver is used to transmit and receive radio frequency communication signals to and from subscriber units located randomly over a wireless communications system coverage area. In order to maintain a reliable communications link with each subscriber unit, the transceiver may transmit a composite signal having a different amount of power individually selected for each subscriber unit. For example, with reference to FIG. 1, wireless communications system service area 50 may include a base station or transceiver 52 that serves a plurality of subscriber units 54-62. Some of these subscriber units may be communicating voice data, or other real-time data, as illustrated by subscriber units 54, 56, and 60, while other subscriber units may be communicating non-real-time data, as illustrated by subscriber units 58 and 62. As used herein, "real-time data" may be defined as data that requires a guaranteed time of arrival, or data that may not be useful if it has been delayed by more than a predetermined amount of time. Real-time data may include digitized voice, video data, data from sensors used to control operations in real-time, and other similar data. Non-real-time data may include data from the internet that represents web pages or other files transferred during a file transfer protocol (FTP) session, audio or video data intended for play back once an entire file has been transferred, and other similar data.
As shown in FIG. 1, subscribers 54-62 may each require a different transmit power, where such power is allocated at transceiver 52. One of the factors that influences the power allocated to a subscriber unit is the distance from transceiver 52. As shown in FIG. 1, the power allocated to subscriber unit 54 would typically be less than the power allocated to subscriber unit 60, which is on the edge of wireless communications system service area 50. And compared to one another, subscriber unit 62 will be allocated more power than subscriber unit 58 because of the difference in distance to transceiver 52.
Another factor that influences the amount of power allocated to a subscriber unit is the presence of an attenuating object in a propagation path between the subscriber unit and the transceiver. For example, in FIG. 1, building 64 lies between subscriber unit 56 and transceiver 52. Building 64 attenuates the signal such that transceiver 52 must allocate more power to subscriber 56 in order to insure reliable communication.
A third factor that influences the power allocated is the data rate of the communications link. Generally, subscriber units in a voice call require less power allocation than subscriber units receiving non-real-time data. This is because the non-real-time data is preferably transmitted at a much higher data rate, and therefore needs a higher power allocation to maintain the required energy per bit at the receiving subscriber unit. A typical bit rate for a voice call is 4 Kbps while a bit rate for a non-real-time data call may be 64 Kbps.
In a communications system where the number of subscriber units is variable, and the distance from the transmitter is variable, and the type of data and location of attenuating objects is variable, a transceiver may in some instances be asked to transmit more power than it is capable of transmitting. Thus, each transceiver has a maximum transceiver power, which should not be exceeded. In the prior art, transmitting above the maximum transceiver power was avoided by limiting the number of subscriber units simultaneously served by the transceiver. This limit on the number of users, coupled with the selection of a larger power amplifier, provided a design margin and a statistical expectation that the maximum transceiver power would not be exceeded. This solution is not optimal because the design margin is capacity that could be used to make money for the system operator without damaging the transceiver.
Other methods of protecting the transceiver include measuring the temperature of temperature sensitive components in the transceiver and shutting down the transceiver if these components exceed a maximum temperature. The solution is undesirable because many calls may be dropped and subscribers frustrated while the transceiver cools and returns to a safe operating temperature.
Therefore, it should be apparent to those skilled in the art that a need exists for an improved method and system for allocating transmit power in a transceiver that serves multiple subscriber units in a wireless communications system, wherein the power allocation scheme protects the transceiver while preserving as much subscriber service as limits will allow.